I recently noticed on my marshall amp an additional audio jack port thats beside the port for the headphone jack that i use my headphones with, I wonder what it is used for does anyone have an answer? 

the amplifier in question is the marshall MG10G

Hello, I've recently bought a DSL20 combo.

On the back there is a small input for external audio devices. In the description it reads:  "AUDIO IN  connect an external audio device here for practice or to jam along with music".

Well, I've tried to connect my phone, which I think qualifies as an audio device, to play backing tracks from youtube.  I was hoping the music would come through the amps speaker and I could    "jam along".

IT DIDN´T WORK!!!

Can anyone explain to me why this is?